Update Priority Modes segment summary

* Display active modes regardless of DND status
* Only consider actually automatic rules for the "x rules can activate automatically" text.
* Also removed the duplicate settings observer in favor of ZenSettingsObserver.

Bug: 355615361
Test: atest ZenModesSummaryHelperTest
Flag: android.app.modes_ui
Change-Id: I39acb785e7b27c95ed664261640c42032acca9d1